This application is related to commonly assigned, co-pending U.S. application Ser. No. 07/956,229, entitled "Complementary Method for Detecting Misfires in Internal Combustion Engines", filed Oct. 5, 1992.
The present invention relates in general to detecting misfires occurring during normal in-use vehicle operation of internal combustion engines, and more specifically to identifying the occurrence of individual misfires in each engine cylinder.
Catalytic converters are used in automobiles to reduce the amount of pollutants in the engine exhaust. When a cylinder misfires so that no combustion or incomplete combustion occurs, uncombusted fuel is introduced into the exhaust which then burns in the hot catalytic converter. The added heat from fuel burning in the catalytic converter tends to destroy the catalyst. Thus, it is desirable to detect and count engine misfires and signal the operator of the vehicle upon occurrence of excessive misfires so that steps may be taken to protect the catalytic converter.
Various methods have been employed in the prior art to detect misfires, such as monitoring of engine crankshaft accelerations, monitoring of electrical properties of the ignition spark and monitoring of various properties of the exhaust gas, including exhaust gas pressure and exhaust gas temperature.
Each prior art technique has certain advantages and limitations. Specifically, prior art techniques have suffered from inadequate coverage of the entire speed-load range of an in-use vehicle engine. Furthermore, the confidence in any detection of a misfire in a cylinder varies depending upon numerous factors, such as signal-to-noise ratio and processing limitations.
To improve misfire detection accuracy and to reduce the occurrence of false indications of misfire (i.e., false alarms), it is known to use multiple misfire detectors operating simultaneously using different detection techniques such that coverage of the speed-load range increases. Prior attempts to combine multiple detection techniques, however, have defined certain regions within the speed-load map of an engine in which each respective misfire detection technique is taken as valid (i.e., will be used in the determination of misfire) and other regions where it is taken as invalid (and is not used in the determination of misfire). In other words, the signal-to-noise ratio of measurements made by a particular misfire technique are treated as though they are constant throughout the entire valid region. However, since the signal-to-noise ratio does in fact vary within the valid region, inaccuracies are introduced into the misfire determination.